1. Field of the Invention
The present invention relates to a method for fixing toner onto a transfer material in image-forming devices such as copiers, printers and facsimile machines. More precisely, the present invention relates to a method for fixing toner onto a transfer material, using a belt-like heating medium.
2. Description of the Related Art
In monochromatic image-forming devices such as ordinary copiers, printers and facsimile machines, a latent image is formed on an image-carrying medium such as a photoconductor drum or belt in a latent image-forming unit, then the latent image is developed in a developing unit to form a toner image, and the toner image is thereafter transferred onto a transfer medium such as transfer paper represented by plain paper, special paper and the like, and fixed thereon in a fixing unit.
On the other hand, in color image-forming devices such as color copiers, color printers and color facsimile machines, in general, plural latent images are formed on an image-carrying medium such as a photoconductor drum or belt, then the latent images are developed with toners of different colors in a developing unit to form toner images of different colors, thereafter the toner images of such different colors are transferred, as combined into a full color image, onto a transfer medium such as transfer paper, and the color image on the transfer medium is then fixed thereon in a fixing unit.
With the recent trend in the art toward energy-saving and compact devices, a fixing unit utilizing a belt-like heating medium (hereinafter referred to as xe2x80x9cbelt fixingxe2x80x9d) has become widely used.
The belt fixing unit of this type makes it possible to extend the contact time of toner to the belt surface at the time of toner fixing, enabling toner fixing at a lower temperature, while toner fusion on the belt often proceeds excessively and leads to offsetting caused by melted toner adhered to the belt surface. In particular, offsetting tends to be caused with color toners since the color toners are generally required to exhibit glossiness to some extent for forming sufficiently transparent and high-quality images, and accordingly, the molecular weight distribution curve of the binder resin contained in the color toner is sharpened so that the color toner may have a sharp melting property. As a result, toner fusion at the time of image fixing proceeds, thereby causing offsetting.
In addition, additives to toner and other components may adhere to the belt-like heating medium, and the belt may be thereby worn or cracked to cause hot offsetting. Also, the belt may be thereby ruptured.
To solve the problems abovementioned, various proposals have been made for improving fixing units and toners for electrophotography.
On the other hand, oil is applied to fixing rollers in fixing units for preventing offsetting. In color image-forming devices, toners of different colors of low viscosity must be used in obtaining images with high priority in glossiness and transparency of the color, and it is indispensable to apply oil to fixing rollers to prevent offsetting.
It is known to add a releasing material (to support fixing) such as wax to toner for preventing offsetting in image fixing. However, depending on the properties of the wax used, especially when the wax added could not well disperse in toner, the wax will separate from the surfaces of toner particles or will bleed out of them in long-term use in developing units, and will stain the surfaces of carrier particles in two-component developers to thereby often worsen the charging properties of toner. In one-component developers, the wax added to toner will adhere to development rollers and to blades used for thinning toner layers, and will therefore often obstruct uniform development with toner. From the viewpoint of development, therefore, preferably an amount of wax to be added to toner is as small as possible.
Known references relating to toner and fixing units are mentioned below, and the structures in the related art disclosed therein and the problems with them are discussed.
Japanese Patent Application Laid-Open (JP-A) No. 07-219274
Configuration:
A color toner which contains a pigment-dispersed resin prepared by adding polyolefin wax and aqueous paste of pigment to a resin solution for pigment dispersion followed by heating the resulting mixture, and in which the SP value difference between the binder resin and the pigment-dispersed resin is in the range of 1.5 to 0.5.
Problem:
This is to improve the dispersibility of polyolefin wax in the binder resin in the toner. However, the polyolefin wax is not sufficiently effective for improving the offset resistance of the toner.
Japanese Patent Application Laid-Open (JP-A) No. 07-311479
Configuration:
A fixing roller for use with the toner described in Japanese Patent Application Laid-Open (JP-A) No. 07-219274, of which the surface layer is an elastic layer coated with fluororesin.
Problems:
Same as above.
Japanese Patent Application Laid-Open (JP-A) No. 07-333903
Configuration:
A toner containing a polyester resin with no THF insolubles having Mn in the range of 2500 to 3500 and Mw in the range of 50,000 to 300,000.
Problem:
This requires a large amount of fixing oil.
Japanese Patent Application Laid-Open (JP-A) No. 07-333904
Configuration:
A toner which contains wax and a polyester resin of which the amount of THF insolubles contained therein is in the range of 15 to 40% and polyhydric alcohol component is specifically defined. In the toner, a difference in refractive index between the binder resin and the wax is specifically defined.
Problem:
Difficult to form glossy images due to a large amount of THF insolubles contained in the toner.
Japanese Patent Application Laid-Open (JP-A) No. 08-50367
Configuration:
A wax-containing toner, in which the wax is an ester wax having a peak value in the molecular weight range of 350 to 850 and 900 to 4000 and having Mn of 350 to 4000 and Mw of 200 to 4000.
Problem:
Merely by defining the property of the wax contained in the toner should not achieve sufficient offset resistance of the toner.
Japanese Patent Application Laid-Open (JP-A) No. 08-50368
Configuration:
A toner which contains an ester wax in which the content of ester compounds having the same total number of carbon atoms is in the range of 50 to 95% by weight. The ester wax contains 3 to 40 parts of ester compounds with respect to 100 parts by weight of the binder resin contained in the toner.
Problem:
Merely by defining the property of the wax contained in the toner should not achieve sufficient offset resistance of the toner.
Japanese Patent Application Laid-Open (JP-A) No. 03-39971
Configuration:
A color toner which contains a resin with no toluene insolubles, in which the resin has a peak value in the molecular weight range of 500 to 2000 and 10,000 to 100,000, and has Mw of 10,000 to 80,000 and Mn of 1500 to 8000 while satisfying Mw/Mn greater than 3.
Problem:
Sufficient offset resistance of the toner is not attained.
Japanese Patent Application Laid-Open (JP-A) No. 04-57062
Configuration:
A color toner having a flow tester melt viscosity of 105(105) poises and a softening point of 90 to 120xc2x0 C., which contains a resin having Mw of 15,000 to 50,000 and Mn of 2000 to 10,000 while having Mw/Mn of 5 to 15.
Problem:
Sufficient offset resistance of the toner is not attained.
Japanese Patent Application Laid-Open (JP-A) Nos. 06-318001 and 08-137306
Problem:
In the fixing units disclosed in these publications, the toner images formed are kept exposed to preheating in the atmosphere. In low-speed image forming, the units are effective for improving the toner fixing and offset resistance, but in high-speed image forming, they are ineffective. In these units, in addition, the distance between the fixing belt B and the recording medium G must be narrowed for getting the benefit of the preheating therein. Thus, there is a need to provide countermeasure against risk of non-fixed toner images rubbing the belt.
Japanese Patent Application Laid-Open (JP-A) No. 04-273279
Problem:
The fixing device disclosed in this publication comprises a step for fixing in which a toner image is fixed onto transfer paper via a belt, followed by a step for carrying the transferred image on the belt and a step for releasing the toner image and the transfer paper from the belt. In this device, therefore, the transfer paper bearing a toner image in the step for fixing is kept preheated by the belt, and, as a result, the toner on the transfer paper tends to be excessively fused to cause offsetting. Moreover, the temperature on the image-bearing surface of the transfer paper tends to be uneven, and accordingly causes uneven image glossiness.
Japanese Patent Application Laid-Open (JP-A) No. 11-44969
Problem:
This publication proposes a toner for electrophotography, which contains a non-linear polyester binder resin and a releasing agent. When used in low-speed processors, the toner exhibits good fixing, glossiness and transparency, but in high-speed processors, the toner is still not on a satisfactory level and needs further improvement.
In reviewing the problems abovementioned, the first object of the present invention is to provide a method for fixing toner to achieve good image quality, stable in actual use in terms of toner fixing ability under low temperature, hot offsetting properties in oil-less use, glossiness of color toner, and transparency of the toner with use in OHP. The object is achieved by thorough examination of the belt fixing unit introduced in the fixing portion and by clarifying interactive properties and effective rage of the interactive properties of the toners used in the method for forming images which utilizes the fixing portion.
The second object of the present invention is to provide a method for fixing toner for forming stable and high-quality toner images on image fixing material (onto which images are fixed) which effectively prevents rubbing of non-fixed images or wrinkles on the image fixing materials, and which also allows smooth transportation and fixing of image fixing materials specifically in high speed fixing device or color image forming device.
The objects abovementioned can be attained by the aspects of the present invention described hereinafter.
The first aspect of the method for fixing toner of the present invention in a belt fixing unit having a fixing roller; a heating roller; an endless fixing belt binding the heating roller and the fixing roller under tension therebetween; a pressure roller disposed to face the fixing roller intervening the fixing belt; and a fixing heater disposed inside one of the pressure roller and the heating roller, comprises: a step for fixing a toner image on an image fixing material by conveying the toner image on the image fixing material in between a portion of the fixing belt not touching the fixing roller and the pressure roller; wherein the toner particle is spherical having a roundness of 0.96 or more and contains a colorant and resin component containing a modified polyester resin.
The second aspect of the method for fixing toner according to the first aspect further comprises, in addition to the step for fixing in the first aspect, a second step for fixing the toner image on the image fixing material by moving the portion of the fixing belt not touching the fixing roller to touch the fixing roller and conveying the toner image on the image fixing material in between the portion of the fixing belt and the pressure roller; wherein a pressure exerted in between the fixing belt and the pressure roller in the step for fixing is reduced to avoid causing wrinkles on the image fixing material.
The third aspect of the method for fixing toner according to the second aspect is characterized in that the fixing pressure in the step for fixing is 1 kg/cm2 or less and the fixing pressure in the second step for fixing is not lower than the fixing pressure of the step for fixing.
The fourth aspect of the method for fixing toner according to the first aspect is characterized in that the heating roller is a heating roller of low heat capacity.
The fifth aspect of the method for fixing toner according to the first aspect is characterized in that the toner contains a releasing agent.
The sixth aspect of the method for fixing toner according to the fifth aspect is characterized in that the releasing agent is a wax having a melting point of 60 to 120xc2x0 C. and the toner contains 1 to 20% by weight of the wax.
The seventh aspect of the method for fixing toner according to the sixth aspect is characterized in that the wax has a weight-average molecular weight of 400 to 5000 and an acid value of 1 to 30 mg KOH/g.
The eighth aspect of the method for fixing toner according to the fifth aspect is characterized in that the releasing agent particles are relatively uniformly dispersed inside the toner particles, and the toner particles each carrying at least 3 releasing agent particles therein account for 70% or more by number of all the toner particles.
The ninth aspect of the method for fixing toner according to the fifth aspect is characterized in that the releasing agent particles each having a dispersed particle diameter of 0.1 xcexcm to 2 xcexcm account for 70% or more by number of all the releasing agent particles.
The tenth aspect of the method for fixing toner according to the fifth aspect is characterized in that the releasing agent particles are needle-wise dispersed inside the toner particles, and the releasing agent particles having a maximum major diameter of 3 xcexcm or more do not exceed 5% by number of all the releasing agent particles.
The eleventh aspect of the method for fixing toner according to the tenth aspect is characterized in that the releasing agent particles are dispersed inside the toner particles in such a controlled condition that the major diameter of each releasing agent particle is not parallel to the toner particle surface but is oriented toward the inside of the toner particle or is not exposed at the surface of the toner surface.
The twelfth aspect of the method for fixing toner according to the fifth aspect is characterized in that the releasing agent is vegetable wax having a molecular weight of 400 to 2500 and an acid value of 1 to 30 mg KOH/g.
The thirteenth aspect of the method for fixing toner according to the fifth aspect is characterized in that the releasing agent is ester wax and the needle penetration at 50xc2x0 C. into the toner particles with the releasing agent dispersed therein is 3 or less.
The fourteenth aspect of the method for fixing toner according to the first aspect is characterized in that the toner is prepared by dissolving or dispersing a toner composition that contains at least a modified polyester resin and a colorant, in an organic solvent, followed by granulating the composition in an aqueous medium.
The fifteenth aspect of the method for fixing toner according to the fourteenth aspect is characterized in that the toner composition further contains a releasing agent
The sixteenth aspect of the method for fixing toner according to the first aspect is characterized in that the toner is prepared by dissolving or dispersing a toner composition that contains at least a polyester prepolymer and a colorant, in an organic solvent, followed by granulating the composition in an aqueous medium while an urea bond-having polyester is formed in the resulting toner particles and while the binder resin in the toner particles being formed is subjected to polyaddition.
The seventeenth aspect of the method for fixing toner according to the sixteenth aspect is characterized in that the toner composition further contains a releasing agent.
The eighteenth aspect of the method for fixing toner according to the first aspect is characterized in that the toner further contains a non-modified polyester resin and the ratio by weight of the modified polyester resin to the non-modified polyester resin in the toner is 5/95 to 80/20.
The nineteenth aspect of the method for fixing toner according to the first aspect is characterized in that the resin component is so controlled that its tetrahydrofuran-soluble component having a molecular weight of 1000 or less accounts for 5% or less in the molecular weight distribution of tetrahydrofuran solubles in the resin component
The twentieth aspect of the method for fixing toner according to the first aspect is characterized in that the resin component has a glass transition point of 55 to 70xc2x0 C.
The twenty-first aspect of the method for fixing toner according to the first aspect is characterized in that the tetrahydrofuran insolubles in the resin component account for 1 to 15% of the resin component.
The twenty-second aspect of the method for fixing toner according to the first aspect is characterized in that the resin component is so controlled that a molecular weight peak of tetrahydrofuran solubles appears in the range of 1000 to 30,000 in the molecular weight distribution of tetrahydrofuran solubles in the resin component and the tetrahydrofuran soluble component having a molecular weight of 30,000 or mope account for 1% or more in the soluble molecular weight distribution, and the toner has a weight-average particle diameter of 3 to 10 xcexcm.
The twenty-third aspect of the method for fixing toner according to the first aspect is characterized in that the molecular weight distribution of tetrahydrofuran solubles in the resin component is arranged to have the number-average molecular weight of the solubles ranging from 2000 to 15,000 and the ratio of weight-average molecular weight/number-average molecular weight to be 10.0 or less.
The first aspect of the present invention provides a method for fixing toner rapidly and stably under high-speed processing or color image-forming.
The second aspect of the present invention provides a method for fixing toner according to the first aspect, which allows forming of good images, satisfying toner fixing, and hot-offset resistance in high-speed processors.
The third aspect of the present invention provides a method for fixing toner according to the second aspect, which allows smooth transportation of the image fixing material without causing wrinkles, and improving toner fixing in low-temperatures.
The fourth aspect of the present invention provides a method for fixing toner according to the first aspect, which shortens rise time and saves energy, while maintaining toner fixing at low temperatures.
The fifth aspect of the present invention provides a method for fixing toner according to the first aspect, in which the toner contains a releasing agent.
The sixth aspect of the present invention provides a method for fixing toner according to the fifth aspect, used in oil-less belt-fixing devices.
The seventh aspect of the present invention provides a method for fixing toner according to the sixth aspect, which exhibits releasability in a wide range of oil-less fixing.
The eighth aspect of the present invention provides a method for fixing toner according to the fifth aspect, which improves toner releasability and glossiness.
The ninth aspect of the present invention provides a method for fixing toner according to the fifth aspect, which improves transparency of a color toner.
The tenth aspect of the present invention provides a method for fixing toner according to the fifth aspect, which improves toner fixing and glossiness.
The eleventh aspect of the present invention provides a method for fixing toner according to the tenth aspect, which improves toner flowability and charge stability.
The twelfth aspect of the present invention provides a method for fixing toner according to the fifth aspect, which improves toner fixing ability.
The thirteenth aspect of the present invention provides a method for fixing toner according to the fifth aspect, which improves fixing ability of a dry toner.
The fourteenth aspect of the present invention provides a method for fixing toner according to the first aspect, which improves fixing ability, glossiness and transparency of the toner.
The fifteenth aspect of the present invention provides a method for fixing toner according to the fourteenth aspect, which improves toner releasability and glossiness.
The sixteenth aspect of the present invention provides a method for fixing toner according to the first aspect, which improves fixing ability, glossiness and transparency of a color toner.
The eighteenth aspect of the present invention provides a method for fixing toner according to the first aspect, which improves fixing and charge stability of the toner.
The nineteenth aspect of the present invention provides a method for fixing toner according to the first aspect, in which the dry toner has satisfying heat resistance.
The twentieth aspect of the present invention provides a method for fixing toner according to the first aspect, which improves fixing ability of the toner.
The twenty-first aspect of the present invention provides a method for fixing toner according to the first aspect, which improves releasability of the toner.
The twenty-second aspect of the present invention provides a method for fixing toner according to the first aspect, which improves releasability of the toner.
The twenty-third aspect of the present invention provides a method for fixing toner according to the first aspect, which improves toner glossiness.
In addition, when the binder component in the toner is controlled to contain THF-soluble content having a molecular weight of 1000 or less accounts for 5% or less in the molecular weight distribution of THF solubles in the binder component, or when the polyester resin in the toner has a glass transition point of 55 to 75xc2x0 C. and has an acid value of 1 to 30 mg KOH/g, or when the THF-insoluble content of the binder component in the toner is 1 to 15%, the toner may be stably fixed to produce good images. In particular, the color toner forms glossy and transparent images suitable for use in OHP.
Further, when the binder component in the toner is controlled to have a molecular weight peak of THF solubles appears in the range of 1000 to 30,000 in the molecular weight distribution of THF solubles in the binder component and the THF-soluble content having molecular weight of 30,000 or more account for 1% or more in the soluble molecular weight distribution and the toner has a weight-average particle diameter of 3 to 10 xcexcm, or when the molecular weight distribution of THF solubles in the binder component in the toner is arranged to have number-average molecular weight of the solubles 2000 to 15,000 and the ratio of weight-average molecular weight/number-average molecular weight at 10.0 or less, then a dry toner which covers a wide range of image fixing is obtained.
Moreover, in the present invention, since the proportion of wax existing on the surfaces of the toner particles can be reduced and, in addition, wax is finely dispersed, the toner forms good color images of high transparency. For these reasons, the toner of the present invention allows designing of compact and inexpensive color copiers and printers without a need to provide oils.
Furthermore, the toner forming method of the present invention realized dispersion of a fine releasing agent inside the toner particles which was unattained by conventional premix grinding methods. In toner particles prepared through suspension polymerization, wax is enclosed in the resin. Therefore, as compared with ground toner, an effect of releasability of the toner prepared by the suspension polymerization with respect to contained amount of wax deteriorates. Still another advantage of the toner of the present invention is that it can readily disperse even in a polyester resin, which, however, could not be used as a binder resin in ordinary polymerization to form toner. In addition, the profile of the toner of the present invention is easy to control, and showing favorable properties of the powdered toner. The present invention has made it possible to form a toner having high transfer efficiency.
According to the toner forming method of the present invention, wax can be finely dispersed in the toner formed, as compared with ground toner. In an ordinary method of grinding toner, fine toner particles having a particle diameter of 4 to 6 xcexcm are substantially difficult to produce in view of the productivity and the production costs. Despite of the situation, the present invention has made it possible to easily produce such fine toner particles. The unit size of wax particles to be dispersed inside the toner particles in the present invention can be reduced, and therefore, the toner particles can form color images of higher quality, especially those of higher transparency suitable for use in OHP. Combining the toner and the belt-fixing device in the present invention solves the problems mentioned below which was not solved in conventional oil-less belt-fixing devices.
1) As compared with roller fixing, the surface pressure in belt fixing is difficult to increase, and the releasing agent is hardly released from the toner in oil-less devices. Therefore, in high-speed oil-less fixing, the releasability only covers a narrow range.
2) Since the surface pressure in belt fixing is low, the color images formed has less glossiness.
3) Since the nip width is broad in belt fixing, often causes wrinkles.